Progress of the technology utilizing membranes having selective permeability has been remarkable in recent years. Such membranes are used in practice in an extensive field as a gas-liquid separation filter and as hemodialysis equipment, blood filter equipment, and a filter for selective separation of blood components in the medical field.
As materials for the membrane, polymers such as cellulose (regenerated cellulose, cellulose acetate, chemically modified cellulose, etc.), polyacrylonitrile, polymethylmethacrylate, polysulfone, polyethylene vinyl alcohol, polyamide, and the like have been used.
Of these polymers, polysulfone-based polymers have attracted attention as materials for semipermeable membranes due to their characteristics of improving hemocompatibility if the membranes are produced from a raw material polymer solution comprising a hydrophilicity promoter, in addition to the excellent heat stability, acid resistance, and alkali resistance.
Membranes must be dried to fabricate a module by causing the membranes to adhere. Porous membranes made from an organic polymer, particularly dialysis membranes and ultrafilter membranes made from a hydrophobic polymer such as polysulfone-based polymer and the like, are known to exhibit significantly reduced water permeability when dried after preparation as opposed to before drying. For this reason, the membranes must always be handled under wet conditions or under the conditions in which the membranes are dipped in water.
The countermeasure that has conventionally been adopted is a method of filling vacant pores in prepared porous membranes with a low-volatile organic liquid such as glycerol before drying. However, since a low-volatile organic liquid is generally highly viscous, removing such an organic liquid by washing the membrane is time-consuming. After washing modules formed from the membrane, eluted substances originating from the low-volatile organic liquid (various derivatives produced by chemical reactions with the low-volatile organic liquid) were existed in a module enclosure solution.
As a method for drying without using a low-volatile organic liquid, JP-A-6-277470 discloses a method of using an inorganic salt, such as calcium chloride and the like, instead of the low-volatile organic liquid. However, the method still requires washing to remove the inorganic salt. Anxiety remains about an adverse effect that the remaining inorganic salt, even in a very small quantity, may have on dialysis patients.
JP-A-8-52331 and JP-B-8-9668 disclose hydrophilic membranes containing polyvinyl pyrrolidone dried without using a low-volatile organic liquid. The patent specifications describe performance of the membranes for separating plasma components from blood. Permeability of plasma proteins, however, indicates that the membranes do not exhibit dialysis performance. In addition, since the patent specifications do not describe the membrane forming conditions, a third party cannot reproduce the membranes by experiments. The membrane structure itself is also not clear. In addition, the membranes are heated in the drying step at a temperature causing decomposition or denaturation of polyvinyl pyrrolidone. The method is thus extremely undesirable from the viewpoint of decreasing elution from the membranes.
JP-A-6-296686 discloses a hollow fiber membrane of which the inner surface of the membrane directly coming into contact with blood has a polyvinyl pyrrolidone content of about 20–50%. This hollow fiber membrane is mainly to provide a wet membrane to which blood proteins, platelets, and the like adhere in a reduced amount. Therefore, a low-volatile organic liquid such as glycerol must be adhered to prevent a decrease in performance due to drying. The module manufactured from the resultant membrane still contains components that allow elution. In addition, the patent specification does not disclose at all the dialysis performance such as low albumin permeability.
Furthermore, JP-A-2000-300663 and JP-A-2001-205057 disclose methods for manufacturing hollow fibers without using a membrane pore holding agent. The patent specifications do not describe dialysis performance of the resulting membranes. The technological correlation between the manufacturing methods and the characteristics of resultant dry membranes is not clear in these patent specifications.